Maximum destructive effect of anti-aircraft projectiles and the like occurs if the projectile detonates within the target. Because the velocity of the detonation products has a component determined by the velocity of the projectile prior to detonation, the precise delay time between target impact (or skin penetration) and detonation should ideally be a function of the projectile velocity. More accurately, since the proper frame of reference for determining maximum destructive effect is that of the target, the delay time should be determined by the relative velocity between the target and the projectile. Furthermore, since impacts at extremely low angles of incidence will result in shallow target penetration or no penetration, the projectile should respond to this situation by detonating immediately.
It is possible to realize or approximate the above described function in a number of ways. Most accurately, perhaps, a Doppler radar or similar system can continuously measure target distance and relative velocity to adjust the delay time. This would be a highly sophisticated system and would consume large amounts of power, thus being difficult to implement in small caliber projectiles. It is also possible to approximate relative velocity by some measure of projectile velocity. For instance, a time of flight measurement could be made or a velocity versus spin rate comparison could be used. Both methods would require power continuously throughout the flight of the projectile and would be insensitive to target velocity and angle of incidence.